diff --git a/datasette/utils/__init__.py b/datasette/utils/__init__.py
index 17c8702f..1d921f02 100644
--- a/datasette/utils/__init__.py
+++ b/datasette/utils/__init__.py
@@ -1294,6 +1294,11 @@ async def derive_named_parameters(db: "Database", sql: str) -> List[str]:
     return named_parameters(sql)
 
 
+UNSTABLE_API_MESSAGE = (
+    "This API is not part of Datasette's stable interface and may change at any time"
+)
+
+
 def error_body(messages, status):
     """
     The canonical JSON error body used by every Datasette JSON error response:
diff --git a/datasette/views/execute_write.py b/datasette/views/execute_write.py
index b7e8288e..6806e69d 100644
--- a/datasette/views/execute_write.py
+++ b/datasette/views/execute_write.py
@@ -2,7 +2,7 @@ import re
 from urllib.parse import urlencode
 
 from datasette.resources import DatabaseResource
-from datasette.utils import sqlite3
+from datasette.utils import UNSTABLE_API_MESSAGE, sqlite3
 from datasette.utils.asgi import Response
 
 from .base import BaseView, _error
@@ -500,8 +500,6 @@ class ExecuteWriteAnalyzeView(BaseView):
                 )
             )
         sql = request.args.get("sql") or ""
-        return _block_framing(
-            Response.json(
-                await _execute_write_analysis_data(self.ds, db, sql, request.actor)
-            )
-        )
+        analysis = await _execute_write_analysis_data(self.ds, db, sql, request.actor)
+        analysis["unstable"] = UNSTABLE_API_MESSAGE
+        return _block_framing(Response.json(analysis))
diff --git a/datasette/views/index.py b/datasette/views/index.py
index 8c5534cb..67296cd1 100644
--- a/datasette/views/index.py
+++ b/datasette/views/index.py
@@ -6,6 +6,7 @@ from datasette.utils import (
     await_me_maybe,
     make_slot_function,
     CustomJSONEncoder,
+    UNSTABLE_API_MESSAGE,
 )
 from datasette.utils.asgi import Response
 from datasette.version import __version__
@@ -152,6 +153,7 @@ class IndexView(BaseView):
                 json.dumps(
                     {
                         "ok": True,
+                        "unstable": UNSTABLE_API_MESSAGE,
                         "databases": databases,
                         "metadata": await self.ds.get_instance_metadata(),
                     },
diff --git a/datasette/views/special.py b/datasette/views/special.py
index 6afb6437..28d4d6a1 100644
--- a/datasette/views/special.py
+++ b/datasette/views/special.py
@@ -6,6 +6,7 @@ from datasette.events import LogoutEvent, LoginEvent, CreateTokenEvent
 from datasette.resources import DatabaseResource, TableResource
 from datasette.utils.asgi import Response, Forbidden
 from datasette.utils import (
+    UNSTABLE_API_MESSAGE,
     actor_matches_allow,
     add_cors_headers,
     await_me_maybe,
@@ -295,6 +296,12 @@ class PermissionsDebugView(BaseView):
         response, status = await _check_permission_for_actor(
             self.ds, permission, parent, child, actor
         )
+        if response.get("ok"):
+            response = {
+                "ok": True,
+                "unstable": UNSTABLE_API_MESSAGE,
+                **response,
+            }
         return Response.json(response, status=status)
 
 
diff --git a/datasette/views/stored_queries.py b/datasette/views/stored_queries.py
index d1f151dd..ea49d983 100644
--- a/datasette/views/stored_queries.py
+++ b/datasette/views/stored_queries.py
@@ -2,7 +2,7 @@ from urllib.parse import parse_qsl, urlencode
 
 from datasette.resources import DatabaseResource, QueryResource
 from datasette.stored_queries import stored_query_to_dict
-from datasette.utils import sqlite3, tilde_decode
+from datasette.utils import UNSTABLE_API_MESSAGE, sqlite3, tilde_decode
 from datasette.utils.asgi import Response
 
 from .base import BaseView, _error
@@ -48,7 +48,15 @@ class QueryParametersView(BaseView):
             parameters = _derived_query_parameters(request.args.get("sql") or "")
         except QueryValidationError as ex:
             return _block_framing(_error([ex.message], ex.status))
-        return _block_framing(Response.json({"ok": True, "parameters": parameters}))
+        return _block_framing(
+            Response.json(
+                {
+                    "ok": True,
+                    "unstable": UNSTABLE_API_MESSAGE,
+                    "parameters": parameters,
+                }
+            )
+        )
 
 
 def _query_list_url(path, query_string, *, set_args=None, remove_args=None):
@@ -315,11 +323,9 @@ class QueryCreateAnalyzeView(BaseView):
                 )
             )
         sql = request.args.get("sql") or ""
-        return _block_framing(
-            Response.json(
-                await _query_create_analysis_data(self.ds, db, sql, request.actor)
-            )
-        )
+        analysis = await _query_create_analysis_data(self.ds, db, sql, request.actor)
+        analysis["unstable"] = UNSTABLE_API_MESSAGE
+        return _block_framing(Response.json(analysis))
 
 
 class QueryStoreView(QueryCreateView):
@@ -384,7 +390,12 @@ class QueryStoreView(QueryCreateView):
         assert query is not None
         if is_json:
             return Response.json(
-                {"ok": True, "query": stored_query_to_dict(query)}, status=201
+                {
+                    "ok": True,
+                    "unstable": UNSTABLE_API_MESSAGE,
+                    "query": stored_query_to_dict(query),
+                },
+                status=201,
             )
         self.ds.add_message(request, "Query saved", self.ds.INFO)
         return Response.redirect(self.ds.urls.path(self.ds.urls.table(db.name, name)))
@@ -405,7 +416,13 @@ class QueryDefinitionView(BaseView):
             actor=request.actor,
         ):
             return _error(["Permission denied"], 403)
-        return Response.json({"ok": True, "query": stored_query_to_dict(query)})
+        return Response.json(
+            {
+                "ok": True,
+                "unstable": UNSTABLE_API_MESSAGE,
+                "query": stored_query_to_dict(query),
+            }
+        )
 
 
 class QueryUpdateView(BaseView):
diff --git a/existing-api.md b/existing-api.md
index 8d390f76..2f9996dc 100644
--- a/existing-api.md
+++ b/existing-api.md
@@ -43,7 +43,7 @@ directory: every claim below is based on the route table in `datasette/app.py`
 - Success content type: `application/json; charset=utf-8`
   (`_shape=array&_nl=on` responses use `text/plain`).
 
-### Success envelope
+### Success envelope and stability marker
 
 Every JSON endpoint that returns an object includes `"ok": true` on
 success. `JsonDataView` injects it automatically for dict responses
@@ -52,6 +52,18 @@ autocomplete views add it explicitly. The former top-level-array endpoints
 (`/-/plugins`, `/-/databases`, `/-/actions`) now return objects wrapping
 their arrays (`{"ok": true, "plugins": [...]}` etc.).
 
+JSON endpoints that are **not part of the documented API** include a
+marker key (`UNSTABLE_API_MESSAGE` in utils/__init__.py):
+
+```json
+"unstable": "This API is not part of Datasette's stable interface and may change at any time"
+```
+
+Currently: the homepage (`/.json`, `/-/.json`), `/db/-/queries/analyze`,
+`POST /db/-/queries/store`, `/db/<query>/-/definition`,
+`/db/-/query/parameters`, `/db/-/execute-write/analyze` and the
+`POST /-/permissions` playground response.
+
 ### Error shape (canonical)
 
 Every JSON error response uses one canonical shape, built by `error_body()`
diff --git a/stable-api-recommendations.md b/stable-api-recommendations.md
index d64b3fb8..baac80f7 100644
--- a/stable-api-recommendations.md
+++ b/stable-api-recommendations.md
@@ -378,7 +378,13 @@ Concerns:
 
 ---
 
-## 9. Define stability tiers explicitly (P1 — documentation, not code)
+## 9. Define stability tiers explicitly (P1 — documentation, not code) — partially implemented
+
+> **Status:** undocumented JSON endpoints now self-describe with an
+> `"unstable": "This API is not part of Datasette's stable interface and
+> may change at any time"` key (homepage, queries analyze/store/definition,
+> query parameters, execute-write analyze, permissions playground POST).
+> The written tier documentation remains to be done.
 
 Not everything under `/-/` can or should carry a 1.0 guarantee. Recommend
 shipping 1.0 with an explicit three-tier contract, per endpoint:
diff --git a/tests/test_api.py b/tests/test_api.py
index 17edc0e5..8ce38fad 100644
--- a/tests/test_api.py
+++ b/tests/test_api.py
@@ -43,8 +43,7 @@ async def test_homepage_sort_by_relationships(ds_client):
     response = await ds_client.get("/.json?_sort=relationships")
     assert response.status_code == 200
     tables = [
-        t["name"]
-        for t in response.json()["databases"][0]["tables_and_views_truncated"]
+        t["name"] for t in response.json()["databases"][0]["tables_and_views_truncated"]
     ]
     assert tables == [
         "simple_primary_key",
@@ -252,6 +251,10 @@ def test_no_files_uses_memory_database(app_client_no_files):
     assert response.status == 200
     assert {
         "ok": True,
+        "unstable": (
+            "This API is not part of Datasette's stable interface"
+            " and may change at any time"
+        ),
         "databases": [
             {
                 "name": "_memory",
diff --git a/tests/test_permissions.py b/tests/test_permissions.py
index 32606789..f8d2c808 100644
--- a/tests/test_permissions.py
+++ b/tests/test_permissions.py
@@ -740,6 +740,10 @@ async def test_actor_restricted_permissions(
     }
     expected = {
         "ok": True,
+        "unstable": (
+            "This API is not part of Datasette's stable interface"
+            " and may change at any time"
+        ),
         "action": permission,
         "allowed": expected_result,
         "resource": expected_resource,
diff --git a/tests/test_queries.py b/tests/test_queries.py
index 11828c4e..7f29a3fb 100644
--- a/tests/test_queries.py
+++ b/tests/test_queries.py
@@ -2140,7 +2140,14 @@ async def test_query_parameters_endpoint_uses_get_sql_only():
     )
 
     assert response.status_code == 200
-    assert response.json() == {"ok": True, "parameters": ["name", "id"]}
+    assert response.json() == {
+        "ok": True,
+        "unstable": "{}".format(
+            "This API is not part of Datasette's stable interface"
+            " and may change at any time"
+        ),
+        "parameters": ["name", "id"],
+    }
     assert permission_denied_response.status_code == 403
     assert permission_denied_response.json()["errors"] == [
         "Permission denied: need execute-sql"
diff --git a/tests/test_success_envelope.py b/tests/test_success_envelope.py
index 97cfd72a..68b042e5 100644
--- a/tests/test_success_envelope.py
+++ b/tests/test_success_envelope.py
@@ -112,3 +112,64 @@ async def test_actions_json_is_object(ds_envelope):
     assert data["ok"] is True
     assert isinstance(data["actions"], list)
     assert "view-instance" in {action["name"] for action in data["actions"]}
+
+
+UNSTABLE_MESSAGE = (
+    "This API is not part of Datasette's stable interface and may change at any time"
+)
+
+
+@pytest.mark.asyncio
+@pytest.mark.parametrize(
+    "path",
+    (
+        "/.json",
+        "/-/.json",
+        "/fixtures/-/queries/analyze?sql=select+1",
+        "/fixtures/-/query/parameters?sql=select+:name",
+        "/fixtures/-/execute-write/analyze?sql=delete+from+facetable",
+    ),
+)
+async def test_undocumented_endpoints_report_unstable(ds_client, path):
+    ds_client.ds.root_enabled = True
+    try:
+        response = await ds_client.get(path, actor={"id": "root"})
+    finally:
+        ds_client.ds.root_enabled = False
+    assert response.status_code == 200
+    assert response.json()["unstable"] == UNSTABLE_MESSAGE
+
+
+@pytest.mark.asyncio
+async def test_query_store_and_definition_report_unstable(ds_envelope):
+    store = await ds_envelope.client.post(
+        "/data/-/queries/store",
+        json={"query": {"name": "unstable_check", "sql": "select 1"}},
+        actor={"id": "root"},
+    )
+    assert store.status_code == 201
+    assert store.json()["unstable"] == UNSTABLE_MESSAGE
+    definition = await ds_envelope.client.get(
+        "/data/unstable_check/-/definition", actor={"id": "root"}
+    )
+    assert definition.status_code == 200
+    assert definition.json()["unstable"] == UNSTABLE_MESSAGE
+
+
+@pytest.mark.asyncio
+async def test_permissions_post_reports_unstable(ds_envelope):
+    response = await ds_envelope.client.post(
+        "/-/permissions",
+        data={"actor": '{"id": "root"}', "permission": "view-instance"},
+        actor={"id": "root"},
+    )
+    assert response.status_code == 200
+    assert response.json()["unstable"] == UNSTABLE_MESSAGE
+
+
+@pytest.mark.asyncio
+async def test_documented_endpoints_do_not_report_unstable(ds_client):
+    for path in ("/-/versions.json", "/fixtures.json", "/fixtures/facetable.json"):
+        response = await ds_client.get(path)
+        assert response.status_code == 200
+        assert "unstable" not in response.json()